Hi Vinyasi,
I know this might be a bit off topic. I have tried some experiments to try and discover why Tesla thought "Rusty" soft iron was so brilliant! I have tried solenoid coil winding, unless I did not wind enough turns - 600, very poor response. At HV High Frequency air coil, can get a lot of sparking within the coil. I am thinking that somewhere along the line, this device of Tesla's would have used soft iron, due to its low carbon. However, if I wind a copper coil on top of a soft iron coil I can get reasonable coupling and good spikes.

That's it. Quirky about the iron, to add kilo's to provide extra output, where did the extra kilos go or how where they attached. Just as a coil with a core might be restrictive.